Glasses based on gallium sulfide and/or germanium sulfide have been modified to improve their properties with either alkali metal sulfides or rare earth sulfides. These two families of sulfide glasses have opposite thermal properties. The sulfide glasses modified with alkali metal sulfides have glass transition temperatures (T.sub.g) of less than about 300.degree. C. and glass stability, measured as the difference between Tg and crystallization temperature (T.sub.x), of about 60.degree.-130.degree. C. In this context, T.sub.g is defined as a temperature at which the glass viscosity is 10.sup.13.5 poise. The sulfide glasses modified with alkali metal sulfides are also hygroscopic. The sulfide glasses modified with rare earth sulfides have T.sub.g in excess of about 550.degree. C. but glass stability of only about 50.degree. to 130.degree. C. The low glass stability of the sulfide glasses modified with rare earth sulfides reduces their glass-forming and optical fiber drawing ability. Furthermore, the two families of the sulfide glasses transmit light but only up to about 9 microns.
Pending in Art Unit 1108 is patent application Ser. No. 08/396,292 filed Feb. 28, 1995, on behalf of inventors Harbison, Jewell, Merzbacher and Aggarwal entitled "Alkaline Earth Modified Germanium Sulfide Glass" discloses a sulfide glass and a process for its preparation. The glass comprises, on mol basis, 20-90% germanium sulfide, 0-60% gallium sulfide, and 5-60% of a modifier selected from alkaline earth sulfides, yttrium sulfide, lanthanum sulfide, zirconium sulfide, hafnium sulfide, indium sulfide and mixtures thereof.
The Aitken et al U.S. Pat. No. 5,392,376 discloses gallium sulfide and gallium sulfide/germanium sulfide glasses for use especially in lasers, amplifiers and upconverters. These glasses are alleged to have excellent transmission far into the infrared region of the electromagnetic radiation spectrum. The gallium sulfide (Ga.sub.2 S.sub.3) glasses disclosed by the Aitken et al patent have the following composition in mole percent:
______________________________________ Ga.sub.2 S.sub.3 glasses ______________________________________ Ga.sub.2 S.sub.3 40-80% RS.sub.x 0-35% Ln.sub.n S.sub.3 1-50% MS.sub.x 1-45% Cl/F 0-10% ______________________________________
where R can be aluminum, tin, arsenic, germanium or indium; Ln can be a rare earth or yttrium; and M can be barium, cadmium, calcium, lead, lithium, mercury, potassium silver, sodium, strontium, thallium or tin. On a ternary component graph of FIG. 1 where GeS.sub.2 is at the apex "x", Ga.sub.2 S.sub.3 and/or In.sub.2 S.sub.3 is at the right corner "y" and modifier M is at the left corner "z", the gallium sulfide glasses disclosed by the Aitken et al patent are represented in FIG. 1 by the 6-sided polygon "B" where the (x, y, z) coordinates for the six points f, g, h, i, j and k defining the polygon are as follows:
f=(35, 64, 1) PA1 g=(19, 80, 1) PA1 h=(0, 80, 20) PA1 i=(0, 55, 45) PA1 j=(15, 40, 45) PA1 k=(35, 40, 25) PA1 a=(94.5, 5, 0.5) PA1 b=(69.5, 30, 0.5) PA1 c=(55, 30, 15) PA1 d=(55, 20, 25) PA1 e=(70, 5, 25)
Although the glasses defined by polygon "B" have high T'.sub.g s, their stability is very poor, as determined by the difference between T.sub.x and T.sub.g, to the point that fibers free of crystals cannot be drawn. Stability of these glasses should be in excess of 150.degree. C. and have appropriate viscosities (about 10.sup.5 poise) below T.sub.x to avoid crystallization on reheating to form highly transparent crystal-free optical fibers.
The gallium sulfide (Ga.sub.2 S.sub.3)/germanium sulfide (GeS.sub.2) glasses disclosed by the Aitken et al patent have the following composition in mole percent:
______________________________________ Ga.sub.2 S.sub.3 /GeS.sub.2 glasses ______________________________________ Ga.sub.2 S.sub.3 5-30% R.sub.2 S.sub.3 0-10% GeS.sub.2 55-94.5% MS.sub.x 0.5-25% Se 0-10% Cl/F 0-25% S/Se 85-125% ______________________________________
where R can be aluminum, antimony, arsenic or indium; and M can be barium, cadmium, calcium, lead, lithium, potassium, silver, sodium, strontium, tin, yttrium or a rare earth. On the ternary component graph described above, the gallium sulfide-germanium sulfide glasses disclosed by the Aitken et al patent are represented in FIG. 1 by the 5-sided polygon "A" where the (x,y,z) coordinates for the five points a, b, c, d and e defining the polygon are as follows:
The glasses defined by polygon "A" have impaired transmission due to formation of crystalline phases upon cooling from the melt.
A sulfide glass is needed that has improved physical and optical properties.